The present invention is in a sintered body based on aluminum titanate which is produced from a mixture which contains oxides of aluminum and titanium. The invention furthermore relates to a process for the production of the sintered body and its use.
DE-AS 27 50 290 (U.S. Pat. No. 4,277,539) discloses a silicatic aluminum titanate. The preparation of this known aluminum titanate sets out from 50 to 60 wt.-% of aluminum oxide, 40 to 45 wt.-% of titanium dioxide, 2 to 5 wt.-% of kaolin, and 0.1 to 1 wt.-% of magnesium silicate. The SiO.sub.2 addition is intended to achieve an improved thermal stability of the pure aluminum titanate. However, the reference also states that the addition includes thermal expansion and retards formation of aluminum titanate, so that a higher firing temperature is necessary in order to obtain the same content of aluminum titanate. Also, the MgO component is considered necessary for the improvement of the physical properties. Sepiolite, for example, is proposed for this purpose. This is said to achieve a relatively flat expansion curve, so that the coefficient of thermal expansion is under 1.5.times.10.sup.-6 K.sup.-1 up to 1000.degree. C. As it appears from the examples of that disclosure, the mixing of aluminum oxide and titanium dioxide is performed in the stoichiometric range (Al.sub.2 O.sub.3 :TiO.sub.2 =1:0.78) or with a very slight excess of titanium dioxide. The ratio of Al.sub.2 O.sub.3 :TiO.sub.2 in Example 5 of that disclosure is 1:0.81. The most favourable results are represented by this example.
The teaching of DE-AS 27 50 290 is thus to be interpreted as recommending the highest possible content of aluminum titanate in the finished body.
DE-AS 12 38 376, discloses production of a ceramic substance from Al.sub.2 O.sub.3, SiO.sub.2 and TiO.sub.2. On account of its plasticity, kaolinite is used as the starting component, and also lithium carbonate aluminum hydroxide and other metal oxides. The resulting low strengths of the substance described in DE-AS 12 38 376 are attributed, according to DE-AS 27 50 290, to the lack of a magnesium oxide compound in the starting mixture. These known substances have a coefficient of thermal expansion in the range up to 1000.degree. C. of -0.1 to -0.8.times.10.sup.-6 K.sup.-1.
EP-A 133 021 refers to an aluminum titanate-mullite ceramic composed of 60 to 75 wt.-% of Al.sub.2 O.sub.3, 15 to 35 wt.-% TiO.sub.2 and 1 to 16.5 wt.-% of SiO.sub.2. Additionally proposed are 0.5 to 5 wt.-% of Fe.sub.2 O.sub.3 and/or 0.5 to 5 wt.-% of rare earth metal oxides. The described compositions of the sintered ceramic include mullite contents of 20 to 40 wt.-%, Al.sub.2 TiO.sub.5 contents of 50 to 70 wt.-% and Al.sub.2 O.sub.3 contents of 10 to 12 wt.-%. Also mentioned are oxides of iron, lanthanum and neodymium in contents of more than 3 wt.-% total. The coefficient of thermal expansion is said to be less than 2.5.times.10.sup.-6 K.sup.-1 in the range from room temperature to 1000.degree. C.
EP-A 37 868 relates to a ceramic material of low thermal expansion which is made on the basis of aluminum titanate with the addition of magnesium oxide and iron oxide. The main component of the crystalline material phase is given as a solid solution of magnesium oxide/aluminum oxide/titanium oxide/silicon oxide/iron oxide. A number of compounds can be used as starting substances, e.g., kaolin to obtain aluminum oxide, or magnesium carbonate to obtain magnesium oxide. The achievable minimum flexural strength values of about 5 MPa at room temperature, are estimated to be very low. The coefficient of thermal expansion is not to exceed a value of 2.times.10.sup.-6 K.sup.-1 in the range from 25 to 800.degree. C.
In accordance with U.S. Pat. No. 2,872,726, the addition of chromium oxide, preferably in amounts of 25 to 60 wt.-%, is proposed for a silicate-free material that is made with the use of Al.sub.2 O.sub.3 and TiO.sub.2. The achieved flexural strengths are relatively high and are far superior to the strengths which are commonly known for aluminum titanate materials. The ratio of aluminum oxide to titanium oxide is not mentioned either for the starting mixture or for the composition of the finished material. Only the ratio of oxygen to the individual elemental metal components such as aluminum, titanium and chromium is given.
U.S. Pat. No. 3,534,286 describes a material in which Al.sub.2 O.sub.3 is one of the chief components. A typical composition consists of 75.2 wt.-% Al.sub.2 O.sub.3, 22.8 wt.-% Al.sub.2 TiO.sub.5 and 2 wt.-% SiO.sub.2. The presence of free TiO.sub.2 in the solid material is not mentioned. The material is used for scattering microwaves and is to have a low porosity exemplified as a value of up to 7%.
In U.S. Pat. No. 3,607,343 TiO.sub.2 is used in an amount of 1 to 50 vol.-% to coat Al.sub.2 O.sub.3 particles. The coated particles are used with the addition of a suitable binding agent, e.g., on the basis of a phenolic resin, for coating by the flame-spraying process.
For the preparation of a sinterable aluminum titanate powder it is proposed according to U.S. Pat. No. 3,825,653 to coprecipitate halogenic or alkoxy compounds of aluminum and titanium and to use the coprecipitate, after drying and calcining, for the sintering of aluminum titanate products. The products manufactured in this manner are said to have a thermal expansion coefficient of less than 1.times.10.sup.-6 K.sup.-1 in the temperature range of 25.degree. to 1000.degree. C. The density is said to be, depending on the manufacturing process, 70 to 85% of the theoretical density of 3.73 g/cm.sup.3, i.e., around 2.6 to 3.2 g/cm.sup.3. Mixtures with a ratio of Al.sub.2 O.sub.3 to TiO.sub.2 are given as 1:1, and 1:3 to 3:1. A mixture of silicate compounds is not mentioned. The powder described in the above U.S. patent is used according to U.S. Pat. No. 3,890,140 for the production of melting crucibles for uranium and uranium alloys. The production of the melting crucible by the hot pressing method sets out from an aluminum titanate powder of a size of 10 to 70 microns. A ratio of 50 mol-% each of Al.sub.2 O.sub.3 and TiO.sub.2 is considered suitable.
In U.S. Pat. No. 4,118,240 a composition is described which consists essentially of aluminum titanate with the addition of 1.5 to 10 wt.-% of tin dioxide (SnO.sub.2) and 2 to 3 wt.-% of SiO.sub.2. Instead of SnO.sub.2, rare earth oxides of, for example, lanthanum, cerium and yttrium are used. A synergetic action between SiO.sub.2 and the rare earth oxides and tin dioxide is expected. The amount of TiO.sub.2 used is about 37 and 38 wt.-% as compared with 53 to 55 wt.-% of Al.sub.2 O.sub.3. In the case of a composition described in this patent, of Al.sub.2 O.sub.3, TiO.sub.2 and SiO.sub.2, a coefficient of thermal expansion of 1.2.times.10.sup.-6 K.sup.-1 is achieved in the temperature range of 20.degree. to 1000.degree. C., and a flexural strength at room temperature of (converted) 18 MPa.
In the Federal German publication, H. J. Pohlmann, K. Schricker, D. H. Schuller, Ber. Dt. Keram. Ges., 52 (1975), pages 179 to 183, the properties of the Al.sub.2 O.sub.3 --TiO.sub.2 --SiO.sub.2 system are described, kaolin finding application as a source of SiO.sub.2. As it appears from the structural studies described, these samples contain substantially naught but aluminum titanate as a crystalline phase.
A porous ceramic molding containing more than 80 wt.-% of aluminum titanate, 4 to 10 wt.-% SiO.sub.2, 0.5 to 5 wt.-% La.sub.2 O.sub.3, CeO.sub.2 and/or Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 and TiO.sub.2 is described in U.S. Pat. No. 4,327,188.
DE-AS 25 09 765 describes a wear-resistant, low-friction and corrosion-resistant sintered material based on TiO.sub.2 which contains 1 to 5 wt.-% Al.sub.2 O.sub.3 and 1 to 5 wt.-% SiO.sub.2, balance TiO.sub.2. The material can also contain a maximum of 0.1 wt.-% of alkali and alkaline earth oxide.
According to GDR Patent No. 29 794, good thermal shock resistance is produced by a very low, preferably negatively linear thermal expansion coefficient. The compositions proposed according to this patent for the production of a highly refractory, oxidic material with good thermal shock resistance, are those of MgO--A.sub.2 O.sub.3 --TiO.sub.2 and of MgO--Al.sub.2 O.sub.3 --TiO.sub.2 --SiO.sub.2, wherein the TiO.sub.2 content is from 15 to 75 wt.-%, the Al.sub.2 O.sub.3 content 70 to 35 wt.-% and the contents of SiO.sub.2 and MgO are up to 40 and 20%, respectively. The linear thermal expansion coefficient achievable with the composition of that patent is said to be less than 4.times.10.sup.-6 K.sup.-1 in the range between 20.degree. and 700.degree. C., or it is to be preferably negative or to differ only slightly from 0. The ratios of admixture of Al.sub.2 O.sub.3 to TiO.sub.2 given in the Examples include the broad range of 1:0.7 to 1:1.7, the latter range applying to a silicate-free composition containing 8 wt.-% of MgO.
The last-named disclosure exemplifies the prevailing approach to improving thermal shock resistance of aluminum titanate materials by obtaining the lowest possible thermal expansion coefficient. As it is stated in that patent, the thermal shock resistance is, however, also directly dependent on, among other things, the thermal conductivity, tensile strength and modulus of elasticity. As it appears from the above-mentioned DE-AS 27 50 290, which is believed to be the closest known state of the art, preference is, however, always given to a low thermal expansion coefficient. The disclosed proposals, however, suffer the common disadvantage that the produced sintered articles do not have a thermal shock resistance that is sufficient in every case, so that defects occur either immediately during casting with metal or, under certain circumstances, after the sintered bodies have been in service for an extended period of time. The defects may be initiated by a single contact, e.g., when the article is invested in a molten metal, for example, but do not manifest themselves until later.
It has also been found that the sintered articles in question still have certain differences in quality in spite of careful attention to their physical characteristics, so that premature destruction of the sintered article occurs during its use, even though the stress does not vary. The reasons for this unexpected misbehavior of the sintered articles are not always discernible, but it is assumed that it is the result of certain structural irregularities of the sintered body.
It is thus an object of the present invention to improve the known sintered moldings based on aluminum titanate, and to improve their use in production operations, and especially their stability when invested with molten nonferrous metals in the temperature range below 800.degree. C.